This invention relates generally to the technical field of torque sensors and, more particularly, to torque sensors that can be used, for example, in assisted steering systems of vehicles.
These power steering or assisted steering systems can be of the following different conventional types: pneumatic, hydraulic, hydrostatic, electrical, or electrohydraulic, depending on the type of energy used. The power steering systems conventionally comprise a torque sensor or torque meter, which is mounted in relation with the steering device of the vehicle. This torque meter generates a signal which indicates the braking torque applied by the driver, conventionally by means of the steering wheel, on the steering device of the vehicle. The output signal of the sensor is conventionally addressed to a steering assistance computer which triggers the power assistance, by actuating, for example, an electrical motor, in the case of electrical power steering. Most of the torque metering devices comprise a torsion bar connecting to half shafts. Guides are provided to avoid the effects of interfering bending forces.
It is known that torque applied by pure torsion to a solid cylindrical bar with circular cross section, made of a material with a Young""s modulus E and a Poisson coefficient xcexc results in an isotropic linear elasticity of:   Γ  =                    E                  2          ⁢                      (                          1              +              μ                        )                              ⁢                        π          ⁢                      xe2x80x83                    ⁢                      d            4                          32              -          θ      L      
where d is the diameter of the bar, L the length of the bar and xcex8 the torsion angle. Consequently, the fact of forming an area with reduced cross section in a steering column allows a concentration of torsional deformation in this area, which is used for measuring the torque.
Examples of torque meters with a torsion bar for power steering are described in the following documents: WO-97 08 527, EP-453,344, EP-325,517, and FR-2,738,339 originating from the applicant; as well as in the following documents:
Japanese patent applications published under Nos. JP-50 77 743, JP-57 19 81 71, JP-59 07 58 64, JP-57 08 77 62, JP-59 11 85 77, JP-61 14 674, JP-62 13 43 71, JP-62 09 44 70, JP-63 29 037, JP-63 09 36 73, JP-30 79 473;
European patent applications published under Nos. EP-369,311, EP-396,895, EP-418,763, EP-515-052, EP-555,987, EP-562,426, EP-566,168, EP-566,619, EP-652,424, EP-638,791, EP-673,828, EP-681,955, EP-728,653, EP-738,647, EP-765,795, EP-738,648, EP-770,539, EP-802-107;
International PTC patent applications published under Nos. WO-87/02 319, WO-92/20 560, WO-95/19 557, WO-96/06 330;
French, UK and U.S. Patent applications published under the following Nos.: GB-2,306,641, FR-2,705,455, U.S. Pat. Nos. 4,874,053, 4,907,668, 4,984,474, 5,123,279, 5,394,760, 5,515,736, 5,578,767, 5,585,573, 5,616,849, 5,641,916.
The principal methods for measuring the torque of a rotating shaft, with or without torsion bar, are methods based on an electromagnetic phenomenon, on optical methods, and on electrical methods. The magnetic methods essentially use magnetostriction and the Hall effect. Magnetostriction is defined as the reversible mechanical deformation which accompanies the variation in the magnetization of a ferromagnetic solid. This phenomenon is reversible; that is, a deformation applied to a ferromagnetic material placed in a magnetic field causes a variation in the magnetization (converse magnetostriction). The Hall effect is conventionally defined as the production of an electrical field which is normal with respect to the current density vector in a conductor or semiconductor placed in a magnetic induction field which is normal with respect to the current density vector.
Examples of torque meters which use magnetostriction are described in the following documents: EP-651,239, EP-502,722, EP-288,049, U.S. Pat. Nos. 4,774,464, 5,450,761, 4,933,580, 4,939,435, 3,548,649, 3,587,305. Sensors which use the Hall effect are described in the following documents: FR-2,737,010, FR-2,689,633.
Optical methods for measuring torque are essentially associated with phenomena of interference or the measurement of optical density. Reference can be made, for example, to the following documents: EP-555,987, U.S. Pat. Nos. 5,490,450, 4,676,925, 4,433,585, 5,001,937, 4,525,068, 4,939,368, 4,432,239, FR-2,735,232, FR-2,735,233, WO-95/19 557.
Electrical methods for measuring torque are essentially connected with the capacitive measurement or a measurement of the phase difference between two magnetic encoders which are mounted circumferentially with respect to the axis of torsion. The document FR-2,724,018 pertains to a torque sensor which comprises a device with gauges for measuring elongation. This device with gauges is placed on a measurement body, which is subjected to mechanical tension under the action of a torque. The document EP-442,091 describes an installation for measuring the angle of rotation or the torque of a rotating or fixed element of a machine, comprising a torsion element in the form of a wheel with spokes connected to several measuring elements, at least one spoke of the wheel with spokes being cut so that the parts of the spoke(s) are applied against each other during the displacement by a predetermined flexion of the other spokes. The measuring device uses Foucault currents.
The foregoing illustrates limitations known to exist in present devices and methods. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.
In one aspect of the invention, this is accomplished by providing a torque sensor comprising a first external ring having at least one elastically deformable connection means, a second external ring placed at a distance from the first external ring, and an internal ring, connected to a rotatable column so the internal ring rotates with the column. The first external ring is capable of being connected to a torque means for applying torque to be measured to the column, and the first external ring is connected to the internal ring by the elastically deformable connection means. The second external ring is connected to the internal ring by at least one essentially unstressed means. Measurement means, for measuring a displacement of the first external ring with respect to the second external ring when a torque is applied to the column by the torque means, is mounted on the second external ring for rotation with the column.
The foregoing and other aspects will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawing figures.